How Well Do Fitness Trackers Monitor Heart Rate?

An increasing number of fitness trackers that are worn on the wrist claim to monitor your heart rate without the need for a chest strap, but how accurate are they?

Fitness trackers measure your activity, such as the distance you walk, the number of steps you climb and the calories you burn. Many fitness trackers are worn on the wrist, and several trackers now say they can monitor a user's heart rate. These include the Basis B1, Withings Pulse and the upcoming Pulsense from Epson.

All of these devices use light to measure your pulse. They shine a light into the blood vessels in your wrist, and then detect the changes in blood volume that occur each time your heart beats and pushes blood through your body. Sensors on the device detect how much light your blood vessels reflect back: Less reflected light means a higher blood volume. [Best Fitness Tracker Bands]

Experts say devices that use this technology are pretty accurate at monitoring heart rate, although they tend to be a little less precise when a user engages in intense activity. Interestingly, some of the makers of fitness trackers do not recommend using their devices if you're solely interested in monitoring your heart rate during workouts.

"It's relatively accurate, from what I've seen," said Matthew Lee, an exercise physiologist and professor at San Francisco State University, who studied a wrist heart rate monitor called the ePulse (made by Impact Sports Technologies) in 2011, but is not involved with any companies that make these devices.

"At higher intensities of exercise, it went off a little bit, but it was still reasonable," Lee said. "As the technology continues to get better and better, it's just going to get more and more accurate."

Detecting pulse with light

The technology required to measure a person's pulse with light has been around for quite a while: Devices in hospitals called pulse oximeters (which place a clip on the finger) measure the amount of oxygen in the blood in a similar way.

Using light to measure a pulse is relatively straightforward when a person is at rest, but becomes challenging when the subject moves around. Ambient light, as well as the movement of the person's muscles, tendons and capillaries, can all interfere with the measurements, said Damon Miller, head of marketing at Basis.

For this reason, heart rate monitors for exercise have traditionally used chest straps — which measure the heart's electrical activity — or required users place their fingers over a sensor.

But in recent years, companies have developed algorithms that cancel out a lot of the "noise" generated by people's movements, and allow sensors to measure heart rate even when a person is on the go.

The makers of one sports watch, the Mio Alpha, say that it can accurately measure heart rate during exercise, for example, while someone is running at speeds up to 12 mph (20 km/h), without a chest strap. For the greatest accuracy, users should wear the device snug against the wrist, above the wrist bone, said Mark Gorelick, director of product science and innovation at Mio. The device is 99 percent accurate when compared to readings on chest strap monitor, the company says.

In contrast, Basis says that although its tracker can detect heart rate while the wearer is moving, it should not be used for the kind of heart rate monitoring that some athletes use while training.

"We try to be straightforward and say you might not get a clear signal constantly" while running or doing other types of intense training, Miller told Live Science. "If you're looking to do heart rate training, there're other products out there."

But you can use the Basis device to measure your resting heart rate, or to monitor how your heart rate changes during the day, Miller said. The Mio Alpha is specifically designed to serve as a heart rate monitor, and for use during exercise, while the Basis is intended as an all-around tracker, he said. The Alpha uses brighter lights to detect a pulse, but this reduces its battery life, Miller said.

Epson says that its Pulsense tracker can be used to monitor heart rate during fitness training. This means people can use the device to see if their heart rate is in their "target zone" during exercise, said Anna Jen, director of New Ventures/New Products for Epson America.

Double check your pulse

Clinton Brawner, a clinical exercise physiologist at Henry Ford Hospital in Detroit, said he recommends that people who use any type of heart rate monitor — be it a chest strap or wrist watch — check the device for accuracy a few times by taking their pulse the old fashioned way.

"The individual needs to be smarter than just the device … to be able to see when those numbers may not be correct," Brawner said.

He recommends checking the pulse on the underside of the wrist, counting the beats for about 15 seconds, and multiplying the number by four. Then, see how this number compares to the reading the device gives, Brawner said.

Lee said that when he and his colleagues compared the ePulse to an electrocardiogram (EKG), the tracker's measurements were within five beats per minute of the EKG during rest and slow walking. When subjects were running, the error range went up to seven beats per minute.

"It's not that far off, but it is losing a little bit of accuracy the more movement there is," Lee said.

The accuracy of such devices will vary from person to person, he said. For example, a device may not fit as well if someone has thinner arms, which may affect the device's readings, Lee said.

Rachael has been with Live Science since 2010. She has a masters degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a Bachelor of Science in molecular biology and a Master of Science in biology from the University of California, San Diego.